Organic electroluminescent display and method of manufacturing the same

ABSTRACT

An organic electroluminescent display and a method of manufacturing the same are provided. The organic electroluminescent display includes a substrate, at least a thin-film transistor, which is formed on the substrate, at least an insulation layer, which cover the thin-film transistor, first electrodes, which are formed in a predetermined pattern on a top surface of the insulation layer and to which a voltage is selectively applied through the thin-film transistor, bus electrodes, which are insulated from the first electrodes, a planarization layer, which is an insulation layer and has openings exposing the first electrodes and the bus electrodes, organic layers, which are formed on a top surface of the first electrodes, and second electrodes, which are formed on a top surface of the organic layer and a top surface of the planarization layer and are electrically connected to the bus electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/663,800 filed Sep. 17,2003 now U.S. Pat. No. 6,917,160, now allowed, and claims the priorityof Korean Patent Application No. 2002-57336, filed on Sep. 19, 2002, inthe Korean Intellectual Property Office, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescent display,and more particularly, to an organic electroluminescent display with animproved electrode structure.

2. Description of the Related Art

Generally, organic electroluminescent displays are self-luminescentdisplays which emit light by electrically exciting a fluorescent organiccompound. They can be operated with low voltage and can be made thin. Inaddition, organic electroluminescent displays have advantages, such as awide viewing angle and a fast response speed, that overcome the problemsof liquid crystal displays. Accordingly, they have been noted as nextgeneration displays.

In such organic electroluminescent displays, an organic layer is formedin a predetermined pattern on a substrate made of glass or othertransparent materials, and electrode layers are disposed under and belowthe organic layers.

In organic electroluminescent displays having the above-describedstructure, when positive and negative voltages are applied to theelectrodes, holes are moved from an electrode, to which a positivevoltage is applied, to a luminescent layer via a hole transport layer.Electrons are moved from an electrode, to which a negative voltage isapplied, to the luminescent layer via an electron transport layer. Theelectrons meet the holes in the luminescent layer, thereby generatingexcitons. The excitons make transitions from an excitation state to aground state, thereby provoking fluorescent molecules of the luminescentlayer to emit light. As a result, an image is formed.

The light efficiency of organic electroluminescent displays operating asdescribed above is divided into internal efficiency and externalefficiency. The internal efficiency depends on the photoelectricconversion efficiency of an organic luminescent material. The externalefficiency is referred to as light coupling efficiency and depends onthe refractivity of each of layers included in an organicelectroluminescent display. Organic electroluminescent displays havelower external efficiency than other displays such as cathode-ray tubesand plasma display panels (PDPs). Accordingly, such organicelectroluminescent displays need to be improved in terms ofcharacteristics of displays, such as brightness and a life span.

In the meantime, in organic electroluminescent displays having theabove-described structure, as the size of a panel increases, the lineresistance of an electrode increases, which causes the brightness of animage to be nonuniform. In particular, when an organicelectroluminescent display is driven by a thin-film transistor, the lineresistance of a cathode increases, and thus the above problem occurs.

Japanese Patent Publication Nos. sho 62-172691, sho 63-172691, hei1-220394, and hei 11-283751 disclose conventional organicelectroluminescent displays. These conventional organicelectroluminescent displays are not provided with an element forreducing the line resistance of a cathode and thus still have a problemof nonuniform brightness of an image due to the line resistance.

SUMMARY OF THE INVENTION

The present invention provides an organic electroluminescent display forreducing the line resistance of an electrode so that the brightness ofan image can be prevented from being nonuniform due to the lineresistance.

The present invention also provides an organic electroluminescentdisplay, in which a bus electrode for reducing the line resistance of acathode is formed in a simple structure, thereby reducing manufacturingcost.

According to an aspect of the present invention, there is provided anorganic electroluminescent display comprising a substrate, at least athin-film transistor, which is formed on the substrate, at least aninsulation layer, which cover the thin-film transistor, firstelectrodes, which are formed in a predetermined pattern on a top surfaceof the insulation layer and to which a voltage is selectively appliedthrough the thin-film transistor, bus electrodes, which are insulatedfrom the first electrodes, a planarization layer, which is an insulationlayer and has openings exposing the first electrodes and the buselectrodes, organic layers, which are formed on a top surface of thefirst electrodes, and second electrodes, which are formed on a topsurface of the organic layer and a top surface of the planarizationlayer and are electrically connected to the bus electrodes. The secondelectrodes are made of a transparent material. Thus, the organicelectroluminescent display is very effective when the second electrodesare applied to a structure in which light is discharged through a frontside. The first electrodes and the bus electrodes are made of the samematerial. The bus electrodes are formed on a top surface of theinsulation layer. Light emitted from the organic layers may bedischarged in a direction of the second electrodes

According to another aspect of the present invention, there is alsoprovided a method of manufacturing an organic electroluminescentdisplay, the method comprising, forming at least a thin-film transistoron a top surface of a substrate, forming at least an insulation layer ona top surface of the thin-film transistor, forming first electrodes, towhich a potential is selectively applied through the thin-filmtransistor, and bus electrodes, which are electrically insulated fromthe first electrodes on a top surface of the insulation layer, forming aplanarization layer on a top surface of the insulation layer to haveopenings at positions corresponding to the first electrodes and the buselectrodes, forming organic layers on a top surface of the firstelectrodes, and forming second electrodes on a top surface of theplanarization layer and a top surface of selected organic layers and areelectrically connected to the bus electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a cross-section of an organic electroluminescent displayaccording to an embodiment of the present invention; and

FIGS. 2 through 7 are cross-sections of stages in a method ofmanufacturing an organic electroluminescent display according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 1 shows an example of an active matrix organic light emittingdisplay (AMOLED), as an organic electroluminescent display according toan embodiment of the present invention. Referring to FIG. 1, a bufferlayer 111 is formed on a top surface of a transparent substrate 100. Theorganic electroluminescent display shown in FIG. 1 is largely dividedinto a pixel formation section 110, which includes a first electrode 112for forming a pixel above the buffer layer 111, and a driving section120, which drive electrodes of the pixel formation section 110 usingthin-film transistors (TFTs) and capacitors.

The driving section 120 includes TFTs and capacitors, which are formedon the buffer 111. The TFT includes a P or N type semiconductor layer122, which is formed in a predetermined pattern on a top surface of thebuffer 111; a gate insulation layer, i.e., a first insulation layer 123,which covers the semiconductor layer 122; a gate electrode layer 124,which is formed on a top surface of the first insulation layer 123 so asto correspond to the semiconductor layer 122; a second insulation layer125, which covers the gate electrode layer 124; and a drain electrode126 and a source electrode 127, which are formed on the secondinsulation layer 125 and respectively connected to both ends of thesemiconductor layer 122 through contact holes 126 a and 127 apenetrating the second insulation layer 125 and the first insulationlayer 123. The capacitor 128 includes a first auxiliary electrode 128 b,which are formed on a top surface of the second insulation layer 125 andconnected to the source electrode 127, and a second auxiliary electrode128 a, which is covered with the second insulation layer 125 so as tocorrespond to the first auxiliary electrode 128 b. A third insulationlayer 129 is formed on a top surface of the second insulation layer 125to cover the drain electrode 126 and the source electrode 127.

Here, the shapes of the first, second, and third insulation layers 123,125, and 129 can vary with a state of the TFT. In addition, the numberof the first, second, and third insulation layers 123, 125, and 129 canbe reduced, and they are made of a transparent material. And numbers ofthe TFT and the capacitor may be varied.

In the meantime, the pixel formation section 110 includes the firstelectrode 112, which is formed on a top surface of the third insulationlayer 129 stacked on the transparent substrate 100 and is electricallyconnected to the drain electrode 126. A bus electrode 150 is formed in apredetermined pattern on a top surface of the third insulation layer 129to be electrically insulated from the first electrode 112.

A planarization layer 130, i.e., a fourth insulation layer, is formed onan entire surface of the third insulation layer 129 on which the firstelectrode 112 and the bus electrode 150 are formed. The planarizationlayer 130 has a first opening 131, which exposes a part of the firstelectrode 112, and a second opening 132, which exposes a part of the buselectrode 150. An organic layer 160 is formed on a top surface of thefirst electrode 112 exposed through the first opening 131. A secondelectrode 170 is formed on the organic layer 160 and the planarizationlayer 130 to be electrically connected to the bus electrode 150. Here,the first electrode 112 and the bus electrode 150 are made of the samematerial. It is preferable that the bus electrode 150 is formed in apredetermined pattern so as to reduce the line resistance of the secondelectrode 170.

In an organic electroluminescent display having the above-describedstructure according to the present invention, when a predeterminedvoltage is applied to the first electrode 112 through the drivingsection 120, a voltage is applied to the second electrode 170. Then,holes move from the first electrode 112 to a luminescent layer of theorganic layer 160, and electrons move from the second electrode 170 tothe luminescent layer. The electrons meet the holes in the luminescentlayer, thereby generating excitons. The excitons make transitions froman excitation state to a ground state, thereby provoking fluorescentmolecules of the luminescent layer to emit light. The emitted light isdischarged through a front side (when the second electrode 170 is madeof a transparent material) or a rear side.

In the above-described procedure, since the second electrode 170 iselectrically connected to the bus electrode 150 having a predeterminedpattern, current and voltage flowing through the second electrode 170can be prevented from dropping. Due to this prevention of the drop ofvoltage and current, current and voltage for exciting the organic layer160 positioned between the first and second electrodes 112 and 170 canbe maintained constant. As a result, the brightness of an image can befundamentally prevented from being nonuniform throughout the image.

FIGS. 2 through 7 are cross-sections of stages in a method ofmanufacturing an organic electroluminescent display having theabove-described structure, according to an embodiment of the presentinvention.

Referring to FIGS. 2 and 3, the buffer layer 111 is formed on a topsurface of the transparent substrate 100, and then at least a TFT layer200 is formed on a top surface of the buffer layer 111. The TFT layer200 can be formed by a typical method, and thus detailed descriptionthereof will be omitted.

Referring to FIG. 4, after completion of the TFT layer 200, the thirdinsulation layer 129 is formed on a top surface of the TFT layer 200.The first electrode 112, to which a potential is selectively appliedthrough the TFT layer 200, and the bus electrode 150, which iselectrically insulated from the first electrode 112, are formed on a topsurface of the third insulation layer 129. Here, the first electrode 112and the bus electrode 150 are formed by bringing an evaporation mask(not shown), which has a pattern for the first electrode 112 and apattern for the bus electrode 150, into close contact with the thirdinsulation layer 129 and evaporating a material for the first electrode112 and a material for the bus electrode 150. Since the first electrode112 and the bus electrode 150 can be made of the same material, they canbe simultaneously formed through evaporation. The formation of the firstelectrode 112 and the bus electrode 150 is not restricted to theabove-described embodiment, but any method enabling the first electrode112 and the bus electrode 150 to be formed simultaneously can be used.

As shown in FIGS. 5 and 6, the planarization layer 130, i.e., the fourthinsulation layer, having the first and second openings 131 and 132 atpositions respectively corresponding to the first electrode 112 and thebus electrode 150, are formed on the third insulation layer 129. Theorganic layer 160 is formed on a top surface of the first electrode 112.

As shown in FIG. 7, the second electrode 170 is formed in apredetermined pattern on a top surface of the planarization layer 130and a top surface of the organic layer 160 to be electrically connectedto the bus electrode 150. The second electrode 170 is formed by bringinga mask having the pattern of the second electrode into close contactwith the planarization layer 130 having the first and second openings131 and 132 and performing evaporation.

As described above, line resistance, which increases with an increase inthe size of an organic electroluminescent display, is reduced, where abus electrode electrically connected to a second electrode is formed ona top surface of an insulation layer having a first electrode, therebyaccomplishing uniform brightness of an image.

Although a few embodiments of the present invention have been shown anddescribed, it will be appreciated by those skilled in the art thatchanges may be made in these elements without departing from theprinciples and spirit of the invention, the scope of which is defined inthe appended claims and their equivalents.

1. An organic electroluminescent display comprising: a substrate; athin-film transistor formed on the substrate; an insulation layercovering the thin-film transistor; a first electrode formed on a topsurface of the insulation layer; a bus electrode which is insulated fromthe first electrode; an insulating planarization layer having an openingexposing the first electrode; an organic layer formed on a top surfaceof the first electrode; and a second electrode formed on a top surfaceof the organic layer and electrically connected to the bus electrode. 2.The organic electroluminescent display of claim 1, wherein the secondelectrode is made of a transparent material.
 3. The organicelectroluminescent display of claim 1, wherein the first electrode andthe bus electrode are made of a same material.
 4. The organicelectroluminescent display of claim 3, wherein the first electrode andthe bus electrode are made of metal.
 5. The organic electroluminescentdisplay of claim 1, wherein the bus electrode is formed on a top surfaceof the insulation layer.
 6. The organic electroluminescent display ofclaim 1, wherein light emitted from the organic layer is discharged in adirection of the second electrode.
 7. The organic electroluminescentdisplay of claim 1, wherein the planarization layer further has anopening exposing the bus electrode.
 8. The organic electroluminescentdisplay of claim 1, wherein the second electrode is disposed on a topsurface of the planarization layer.
 9. The organic electroluminescentdisplay of claim 1, wherein the second electrode is directly in contactwith the bus electrode.
 10. The organic electroluminescent display ofclaim 1, wherein the organic layer has an electron transport layer whichcovers the substrate.
 11. The organic electroluminescent display ofclaim 1, wherein the bus electrode is formed in a predetermined patternto reduce a line resistance of the second electrode.
 12. An organicelectroluminescent display comprising: a substrate; a plurality ofthin-film transistors formed on the substrate; a first insulation layerformed with a first surface contacting the thin-film transistors and asecond surface; a plurality of first electrodes formed on the secondsurface of the first insulation layer, each first electrode electricallycommunicating with a respective one of the plurality of thin filmtransistors and having a first surface contacting the first insulationlayer and a second surface; a bus electrode insulated from the firstelectrodes; a second insulation layer formed to cover the firstinsulation layer and having a planarization surface and an opening forat least a portion of each of the plurality of first electrodes; aplurality of organic electroluminescent emitters, each emitter having afirst surface formed on the second surface of a respective firstelectrode and a second surface; and a second electrode formed on thesecond surfaces of the organic electroluminescent emitters andelectrically connected to the bus electrode.